Wood as an Ideal Material for Bow Construction
Development of custom, high-performance materials have yet to yield a base material as ideal for a bow as wood. Wood's properties have been exploited extensively to yield the best possible performance.
The bow is one of the oldest tools ever enlisted by man. Commonly believed to be the natural evolution of hand-launched spears, the bow allows launch speeds far in excess of what a human could achieve through throwing. The bow was the tool of hunting, war, and ceremony for multiple millennia. Through the ages of bow use, bows continually improved, incorporating new technology and new materials as they became available.
While modern bows incorporate high-tech synthetic materials, the performance of the wooden bows the ancients used often exceeds their modern equivalents. Remarkable archers using modern bows can achieve distances of about 850 yards, yet Turkish archers of centuries past have reportedly achieved more than 970 yards [RAUSING 31]. The Turkish achievements represent the epitome of archery, and the path to their accomplishment illustrates the skill and cleverness of thousands of years of bow makers, while simultaneously testifying to the amount of technical ingenuity involved in bow construction.
Wood forms the basis of most bows (with the interesting exceptions including Indian and Chinese steel bows of about 300BC, not discussed in this paper). The basic properties of wood (and their applications to bow making) will be discussed, as will the criteria used to select wood for bows. A technological description of significant innovations will be interspersed, with a focus on those innovation's additional demands on or exploitation of the properties of wood.
Perhaps the most significant characteristic of wood are the anisotropic properties. Wood's strength along the grain can exceed the strength across the grain by more than an order of magnitude, depending on the species. Another characteristic that must be considered is the considerable difference in strength between compressive and tensional loads; individual cells have a tendency to crumple under compression and hence fail more quickly than when under tension.
Heartwood, the older and extractive-rich core of a log, and sapwood, the marginally alive and relatively extractive-free wood, have significantly different properties. Heartwood has moderately improved stress-bearing capability, as well as resistance to decay and often a greater aesthetic appeal. Rausing describes the differences: "[Heartwood] has spring and sharp recovery, [sapwood] forms a natural backing" [RAUSING 153]. The spring and sharp recovery can be attributed to the improvement in Young's Modulus [BODIG 464], but it is unclear which properties Rausing is alluding to with sapwood. While the moisture content of the sapwood and heartwood is initially extremely different, the bow-making process usually involves extensive drying, so differences in mechanical properties due to moisture is not a factor in design.
Preserving the long, continuous fibers of wood is beneficial. Thus, when cutting a stave from a log, the bowyer may cut one edge as parallel to the grain as possible. Most likely this edge would be used for the bow's back, where the bow is under tension and would benefit the most from longer fibers.
Tree rings also degrade the performance of wood. Latewood and earlywood can have massive differences in mechanical performance, due to the anatomical differences in the cells grown in those times. Bowyers select trees that have been well shaded (yielding trees with smaller year-rings and better mechanical performance), and may even discard all but the northern-facing quarter of the tree. The orientation of the rings must also be decided; should they run concentrically with the curved belly, anti-concentrically, or should they be approximately perpendicular?
With these characteristics in mind, consider the physical demands placed upon a bow by the action of the archer. When the bowstring fully displaced, the bow is strongly bent. The bow experiences both compression and tension simultaneously on the belly and back of the bow, respectively. In order to combat this, the grain of the wood is oriented vertically, so that the greatest forces weigh upon the strongest direction of the wood. The force applied to the bow through the bowstring can be up to a couple hundred pounds [RAUSING 161], and consequently, the forces on the bow are considerable.
The force applied through the bowstring, however, is not directly proportional to the useful force applied to the arrow. In the ideal case, all the stored energy of the bow would be simultaneously imparted upon the arrow, but in actuality, the force is applied over an interval of time. This period of time-the time the bow requires to restore its previous shape-is a critical characteristic of the bow's material. It is the shortness of the restoration time of wooden bows that makes them so effective, further enhanced by wood's nearly ideal elastic behavior over a wide range of forces.
Another concern which must be addressed is the relaxation behavior-a material under stress may deform plastically under force. In wood's case, the behavior, while largely elastic, has a time-dependent deformation characteristic. Fortunately, in a bow, the force is applied to wood for only a short period of time and this effect can be largely neglected.
The essential features of a bow is the strip of wood, whose grain is oriented vertically. Since the bow will be bent, it is desirable to make the bow non-circular in cross-section in order to avoid uncontrollable twisting of the bow. Making the cross section asymmetrical yields a bow which can bend in two directions but is utterly rigid in the other. Further, it is desirable for the back of the bow, the side which faces away from the archer, to be flat, so that the strain is distributed across as wide a region as possible [RAUSING 41]. Also desirable is the preservation of long wood fibers on the back of the bow, to better combat the lesser strength under tension. Therefore, in many bows, the back is precisely parallel to the original wood grain.
Early bows consisted simply of a debarked branch and were thicker on one end than on the other. A bow of this type is asymmetrical; the arrow sits nearer the thinner end of wood in order to balance the lack of flexibility in the thicker part. A historically common, perhaps obvious, improvement is to further refine the shape of the bow to make it symmetrical. This allows a more even distribution of stress on the bow, yielding greater overall utilization.
The Siriono of Eastern Bolivia are a surviving Old Stone Age people, documented in the book Nomads of the Long Bow, by A. Holmberg. (Despite the title, the book represents a complete ethnography, with equal attention given to all aspects of life, not merely their bows.) Documented in this ethnography is a description of the process used to manufacture bows. The Siriono individually crafted their own bows from the very hard and black wood that is the heartwood of the chonta palm tree. The hunters demand that the bow be able to tolerate their maximum pull, but the lack of bowyer expert yields many laboriously constructed bows which snap on their first pull despite the effort invested in finding a tree with just the "right" properties.
The bow is cut from a vertical section about four inches wide and as tall as the bow's desired height, which can exceed nine feet. The Siriono are careful to use the inner side of the heartwood as the belly of the bow, and the bark side as the back. This makes the rings of the tree concentric with the curvature of the belly, and is a characteristic common of many bows. If the rings were oriented perpendicularly to the line of fire, the bow would be prone to splitting along the weak yearrings.
Using a mollusk shell as a planing tool, the wood is shaped to yield an oval cross-section whose major axis is about two inches in the middle, and a quarter inch at the extremes. Unfortunately, no information about the size of the minor axis is available. From a photo of the bow, it is apparent that the bow is of a simple, symmetrical design.
As mentioned previously, many mechanical properties affect the performance of a bow. Further, the location of the tree itself, or the perceived quality of the tree (straightness or other cultural criteria) influences which tree is selected for harvest. But what species are generally desirable?
It is known that bowyers would import staves great distances (Rausing suggests that stave and other bow-related materials (such as sinew) might have played a non-trivial role in the development of trade itself). From this it can be discerned that bowyers were highly selective. On the other hand, there seems to be a wide variety of species deemed suitable, based on archeological evidence.
Rausing reports that elm was selected over oak despite oak's generally superior mechanical properties; regularity of grain may be an issue. Herodotus claims the Bactrians, Caspians, and Indians used cane bows, the Ethiopians the stem of the palm, and the Lycians cornel wood. Apollonious refers to bows made of olive. Rausing mentions birch, elm, osage orange, and lemonwood, with yew being particularly desirable.
While early bows consisted of essentially straight staves, later bows offered several refinements. The goal of any bow is to transfer as much energy to the arrow as possible, and to deliver that arrow as accurately as possible. To some extent, these two goals are mutually exclusive, however, changes in the design of the bow helped archers improve their cast and accuracy.
The first bows were the asymmetrical bow, made from a single branch and relatively unmodified from their natural shape. While one arm of such a bow is in its maximum energy state, the other arm would remain well below its capacity. Further, upon release, the energy imparted to the arrow (and to the archer) is asymmetrical; the bow is difficult to aim.
By making the bow symmetrical, each arm can be bent to its maximum state. A symmetrical bow can deliver more power than an asymmetrical bow and be considerably smaller. Symmetrical bows are usually called segmented bows. A reasonably rigid segment bow would have to be quite long in order for the user to achieve a full draw on the bow.
Other designs would improve the draw on a bow while reducing its size. This was accomplished by heating and bending the wood to change the geometry of the weapon. Several bow designs, like the doubly convex and angled bow, succeeded in increasing the pull, but little else.
A major improvement followed with the arrival of reflex bows-- bows which, when unstrung, bent the wrong way. Herodotus takes note of the Arabians in his fifth history, "The Arabians... carried at their right side long bows, which when unstrung bent backwards. In addition to allowing for greater draw in a smaller package, the ears-- the part of the bow which bends the wrong way-- provide a mechanical advantage for launching arrows at high speed. The Turks developed doubly concave reflex bows, which were the bows capable of such fantastic ranges.
But by this point, wood was no longer the sole material used in bow construction. Composite bows had entered the scene. They combined bone or horn and sinew with the wood to complement the wood's capabilities. Bone and horn are excellent in compression, so they were glued to a wooden bow's belly. Likewise, sinew excels in tension, so sinew was applied to the back of the bow. The era of composite bows added many dimensions of possibility to bow design. Throughout the entire history of archery, however, wood has never been replaced as the fundamental base material.